Add Java 8 Optional Api map, flatmap

[sav007]

No description provided.

[digitalbuddha]

filter would be nice too if you're at it :-)

Just to point out this is making us different from Guava Optional so we will have harder time pulling in changes.

[sav007]

@digitalbuddha the real intention of these changes is to have flat map, that will make chain look nicer:

before:

response -> response.data()
        .transform(it -> it.node.get())
        .transform(it -> it.asCheckout.get())
        .transform(it -> it.availableShippingRates.get())

after:

response -> response.data()
        .flatmap(::node)
        .flatmap(::asCheckout)
        .flatmap(::availableShippingRates)